1. Field of the Invention
The present invention relates generally to a cellular communications system and in particular to increase the spectral efficiency of the cellular communications system using fractional reuse through channel allocation tiering (CHAT).
2. Background and Objects of the Present Invention
Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) are widely accepted multiple access communication methods currently adopted in the wireless industry. Particularly, the combination of both multiple access methods, e.g., Global System for Mobile communications (GSM) is the most popular and global wireless communication access method. Due to the rapidly increasing demand and use of mobile radio communications, new services and features are entering the market. These services, e.g., audio and video, require the network to support high bit rate services at the same time as traditional speech services continue to expand. With the acquisition of new sites becoming increasingly difficult, efficient utilization of the limited frequency spectrum is required, at the same ensuring that link quality is maintained at an acceptable level.
To illustrate techniques that enhance spectral efficiency, consider the commonly used Global System for Mobile communications (GSM). GSM operates in either the 900 MHz, the 1800 MHz or the 1900 MHz frequency ranges. Taking for example, GSM operating in the 900 MHz range, there are two frequency bands allocated for uplink and downlink traffic. Each frequency band is frequency divided into a number of frequency carriers, e.g., 124 frequency carriers, each of which utilizes 200 kHz of spectrum. Each carrier is further divided into time slots, e.g., eight time slots. In general, one base station can contain one or more sector cells. In each sector cell, there are a number of communication units, e.g., transceivers.
Radio resource (RR) hopping can be implemented in the system. The best example of RR hopping is frequency hopping. Frequency hopping schemes can be used in the system, in addition to other schemes, to enhance performance and reduce interference, as is well known in the art. In this instance, each of the transceivers may operate at one specific frequency and the signals hop from one transceiver to another, e.g., baseband frequency hopping, or in some cases each transceiver utilizes several frequencies, by hopping between frequencies, and transmits only one signal all the time, e.g., synthesized frequency hopping.
In general, when a service session radio resource is changed through time, interference is reduced because the service session is time and frequency dependent. For example, a dip in the transmission channel due to multipath fading or other factors affecting the channel usually occurs at a frequency in a given instant of time. When the channel frequency hops from one frequency to the next, the dip occurring in the channel will only affect this channel during a part of the dip while at this frequency or even only at a time slot in this frequency. Therefore frequency hopping improves performance, particularly in combination with coding and interleaving. Moreover, planning of cellular networks is done to improve spectral efficiency using a number of methods such as frequency hopping and adaptive antennas. Frequency hopping could be used to reduce the effect of interference in a system, as described hereinabove. Adaptive antennas could also be used, as could a number of other techniques such as link adaptation, transmitter diversity, and advanced receiver algorithms.
The combination of a number of such techniques could increase spectral efficiency to the point where fully loaded one-reuse networks are likely to become feasible in terms of service quality. Taking spectral efficiency beyond fully loaded one-reuse means either introducing complicated statistical multiplexing techniques or reusing the same frequency multiple times within a cell. The latter alternative is simpler and provides a smooth evolution for existing mobile communications systems. If frequencies are to be reused multiple times within a cell, methods are needed to manage the potentially severe intracell interference that could occur. One technique is so-called Spatial Division Multiple Access (SDMA) which utilizes narrow-beam adaptive antennas to allocate the same radio resources to users in different directions from the base station. The drawback with SDMA is that mobility management is complicated, since complex intracell handover and beam management algorithms are needed if users on the same channel are to be kept spatially separated. Moreover, diversity combining from multiple beams can generally not be used since it either introduces intracell interference or further complicates beam management or both. Intracell interference will cause severe performance degradation since there is no interference diversity within the cell and there is no possibility to use interference suppression techniques. This is because RR hopping sequences and codes, where used, are allocated on a per cell basis.
A better option is the simpler technique described hereinbelow, that can raise the spectral efficiency of cellular networks and make fully loaded one-reuse networks effectual in terms of service quality without requiring complicated mobility management and statistical multiplexing procedures. The present invention overcomes the limit of fully loaded one-reuse networks in terms of a lack of available channels without dramatically increasing the complexity of the system. The invention described hereinafter is both a technique to raise the spectral efficiency of cellular networks and a method to enable reuse less than one, also known as fractional reuse.